1. Field of the Invention
This invention relates to a cryostat which houses a superconducting coil, and more particularly to a compact cryostat whose external height is reduced.
2. Description of the Prior Art
FIG. 1 shows a schematic sectional view of a conventional cryostat as disclosed on page 425 of the Proceedings of the 9th International Conference on Magnet Technology, Zurich, Switzerland (1985), wherein a superconducting electromagnet coil 1, for example, is disposed in a bath of liquid helium 4 in the lower chamber 3a of a housing 2. The housing includes a larger diameter upper chamber 3b serving as a reservoir for an additional volume of liquid helium. A tank 5 containing liquid nitrogen 6 is disposed above the housing 2, which is surrounded by a thermal shield 7 made of copper, aluminum or the like. The shield 7 and the nitrogen tank 5 are in turn surrounded by a vacuum vessel 8, and the spaces 9 flanking the shield are evacuated to thermally insulate the assembly.
In the operation of any cryostat it is necessary to periodically replenish the liquid helium supply. To reduce the frequency of such replenishment and thus prolong the operating period of the superconducting coil, the larger diameter upper chamber reservoir 3b is provided.
The liquid helium 4, which is at a very low temperature and has a small latent heat, will easily evaporate upon a slight external heat loss due to thermal conduction. To minimize such evaporation the conventional cryostat is thus provided with the liquid nitrogen tank 6 to thermally insulate the liquid helium together with the attached thermal shield 7.
Such a conventional cryostat construction is undesirably high due to the disposition of the nitrogen tank above the helium reservoir, and is difficult to economically fabricate owing to the stepped configuration of the thermal shield and the vacuum vessel.